This invention relates to an improved transparent laminar structure useful as a window in solar collectors, to improved solar collectors employing the laminar structure and to a method for collecting solar radiation.
Solar collectors serve to convert a major part of the radiation spectrum of sunlight into heat, which can then be transferred to a heat transporting medium, for example, water. An important property of a successful solar collector is the capability of efficient conversion of solar radiation into heat. Significant factors in the overall efficiency are the extent of transmission of solar radiation incident on the glass structure employed as a transparent cover or window and the capability of minimizing loss of heat from the absorber.
Heretofore known heat-insulating, light-transmitting laminar structures proposed for use as a cover or window for a solar collector have included a glass substrate, an infrared-reflecting (IRR) layer provided on at least a portion of a major surface of the glass substrate, and a solar reflection-reducing layer provided on that surface of the infrared-reflecting layer opposite the glass IR layer interface surface thereof. As described in U.S. Pat. No. 4,091,793 (Hermann et al) the transmission by glass tubes of incident solar radiation can be increased by providing on their inner surfaces a reflection-reducing layer of cryolite (Na.sub.3 AlF.sub.6), magnesium fluoride (MgF.sub.2), or an oxide of silicon (SiO.sub.x.SiO.sub.2). As further described therein, the reflection-reducing layer is effectively provided on an infrared reflecting layer, which is provided directly on the surface of the glass tubes for reflecting infrared radiation emitted by the absorber portion of a solar collector employing the tubes separating (i.e. disposed between) the absorber and the ambient atmosphere.
The aforesaid Hermann et al patent teaches that suitable infrared reflecting layers for the glass tubes thereof are made of silver, gold or tin oxide, but are preferably of indium oxide. The absorption surface thereof is taught to be either non-selectively blackened (.alpha.=1, e=1) or is blackened and has a low emission factor (.alpha..gtoreq.0.9; e.ltoreq.0.3) for thermal radiation. The Hermann et al patent is incorporated herein by reference.
However, the solar collector art is still in need of improved light-transmitting laminar structures which are characterized by a good balance of infrared radiation trapping (i.e. good IR reflecting properties) and high solar transmission (i.e. a good coupling of solar conversion efficiency with low heat loss properties when the structure is included in a solar collector wherein the structure is interposed between the absorber and the ambient atmosphere). More particularly, the prior art is in need of a transparent laminar structure suitable for use in a solar collector which overcomes the problem of inherently substantial transmission-reducing Fresnel reflection at the cover/air interface of IR mirrors (glass substrate/IR reflecting layer laminar structures) employing IR reflecting layers made of semiconductor oxides such as tin oxide, indium oxide, etc., especially when such oxides are doped with doping agents, e.g. fluorine (F), tin, antimony (Sb), etc.
It has now been found by practice of the present invention that the foregoing high conversion/low heat loss is achieved by employing, as a reflection-reducing layer or overcoat on the semiconductor oxide layer of an IR mirror, a layer of polymethacrylate composition.